Depicted in FIG. 1, as a non-restrictive example of application of the moems technology, is an axonometric illustration of an optoelectronic device 10, integrated on a die 51 which, by way of non-restrictive example, may be of a semiconductor material, generally silicon. On the upper face 11 of the die 51, etchings 12, needed for instance for the seating of optical fibres, are made. Also on this upper face 11, there are outer pads 52 to which optoelectronic components 53 have been soldered, and inner pads 54 needed for connecting the optoelectronic device to an external circuit not shown in the figures.
Still in FIG. 1, the x, y and z axes which give the three-dimensional references of the die 51 are defined.
The process for manufacturing the optoelectronic device will now be described according to a technique known as “lift-off” to those acquainted with the sector art, by means of which geometries having different layers can be produced directly on non-reactive materials, such as for example titanium, platinum, gold, or with which soldering alloys (for example gold/tin 80/20) may be deposited selectively, or non-metallic materials may be deposited selectively. Reference should be made to the flow diagram of FIG. 2, for only the steps necessary for the understanding of this invention.
In a first step 70, a wafer 66 is made available on which the dice 51 are made (FIG. 3).
In a step 71, illustrated with the aid of FIG. 4, on the dice 51 a layer 61 is spread of “lift off resist”, for instance of the LOR® series by Micro-Chem, having for example a thickness of between 0.5 and 6 □m, as indicated in the cross-section detail of FIG. 4 which concerns an area without etchings. The lift off resist is usually applied in the fluid state, by means for instance of a centrifuge in a process known as “spinner coating”.
In a step 72, on the lift off resist 61, a layer 60 of conventional type positive photoresist is laid, having for example a thickness of between 0.5 and 20 □m, as also indicated in FIG. 4. The positive photoresist is also usually applied in the fluid state, by means for instance of spinner coating.
Designated with number 14 is the upper face of the layer 60 of photoresist, substantially parallel to the face 11 and to the x, y axes.
The photoresist is defined as “positive” if, starting from an initially insoluble state in its development solvent, it depolymerizes due to the effect of radiation, for example ultraviolet, becoming soluble.
The technology described, which uses the layer 61 of lift off resist and the layer 60 of conventional positive photoresist, is called “bilayer”.
In a step 74 (FIG. 5), exposure of the photoresist is performed to ultraviolet (UV) radiation by means of a mask 13 provided with windows 122. The photoresist, being positive, depolymerizes in the zones 26 corresponding to the windows 122, and therefore struck by the radiation UV, whereas it remains insoluble in those zones that are kept in the shade by the opaque areas of the mask.
In a subsequent step 75 the layer of photoresist 60 is developed according to known techniques which, by means of a solvent, remove the photoresist only in the zones 26 depolymerized by ultraviolet (UV) radiation through the windows 122: thus cavities 64 are made, bounded by edges 25 (FIG. 6).
The same solvent dissolves the underlying lift off resist 61 to a greater extent than the photoresist 60, thereby producing sub-etchings 22, the depth of which depends on the development time.
A first alternative exists, illustrated in FIG. 7, according to which only the monolayer layer of photoresist 60 is produced. After development, the cavities 64′ in this case are bound off by walls 15.
On account of diffraction and reflection phenomena in the UV radiation, the depolymerization of the layer of photoresist 60 takes place, parallel to the plane x-y, to a greater extent in the vicinity of the face 11 of the die, and to a lesser extent in the vicinity of the surface 14 of the photoresist: the walls 15 are not therefore parallel to the z axis, but instead have an undercut β, positive according to the sign convention indicated in FIG. 7. This technology is called “monolayer”, and is less expensive but also controllable with much less precision than bilayer.
There is also a second monolayer alternative, again illustrated with the aid of FIG. 7, according to which only the layer of photoresist 60 is produced, which is then treated with a surface modifier, for instance toluene, which renders the upper surface 14 more resistant to the solvent: in this way, after development, in the same photoresist 60, walls 15 with sub-etching or with a more pronounced positive undercut β are produced. Costs and controllability of this second alternative lie between those of the first monolayer alternative and those of the bilayer technology.
In a step 76 a vacuum deposit is made, for instance of a metal, on the surface 14, in which the latter remains intact, and on the face 11 where this is uncovered through the effect of the development described in step 75. The deposit takes the form, for example, of a “sputtering” or “electron beam” process, both of which are known, the result of which is a subassembly 23, illustrated in FIG. 8, comprising a first deposited layer (52, 54) adherent to the face 11, which assumes the geometry of the cavities 64 and effectively constitutes the outer pads 52 and the inner pads 54, and a second deposited layer 16 adherent to the surface 14. No layer is deposited on the sub-etchings 22.
Even when the monolayer technology is adopted, no layer is deposited on the walls 15, thanks to the positive undercut β and to the sub-etchings, if any.
This separation between the two deposited layers is essential for subsequent operations, and is a first fundamental reason for choosing positive photoresist to produce the layer 60, as there is in fact no practical possibility of producing a bilayer with negative photoresist, whereas, if a monolayer with negative photoresist is chosen, the undercut β would be negative according to the sign convention adopted, and the walls of the cavity would be covered over by the deposit.
The deposited layers 52, 54 and 16 may be metallic, or made of non-metallic materials, such as for example oxides, nitrides, carbides and the like.
If the deposited layers 52, 54 and 16 are made of metals, these may be non-reactive type, such as for instance titanium, gold or platinum, for the outer pads 52 or the inner pads 54, or a gold/tin 80/20 alloy for the soldering. These alloys are generally deposited, according to a known technology, in alternate layers of the component metals: the various layers are produced in appropriate ratios between the thicknesses to give the alloy—generally eutectic—the right composition, and can have an overall thickness, for instance, up to 5 μm.
In a step 77, illustrated with the aid of FIG. 9, the layer 60 of positive photoresist and the layer 61 of lift off resist are removed by means of a process known as “lift off” to those acquainted with the sector art. The subassembly 23 is plunged into a solvent 26, for instance acetone which, through the edges 25 and the sub-etchings 22, free of the deposited layer, penetrates through the layers 60 and 61, and dissolves them, as indicated by the arrows 21, eliminating them completely and freeing the second deposited layer 16, which is then cast aside.
The operation is facilitated by a mechanical action such as, for instance, an ultrasound wash, and can only be conducted on a positive photoresist: this is a second fundamental reason why positive photoresist is chosen for the layer 60. If, on the other hand, a negative photoresist were to be chosen, it would not be possible to perform the lift off operation with today's technologies.
Where the monolayer technology is selected, this step 77 is performed in the same way, since the solvent can penetrate through the walls 15, also free of the deposited layer.
At the end of the step 77 the subassembly 23 is finished, as shown in FIG. 10, where a die 51, an outer pad 52 and an inner pad 54 may be seen.
This process, however, has a number of technical problems that will now be described.
When etchings 12 are made on a face of the die 51, as indicated in the section view of FIG. 11, a few tenths or hundredths of a μm thick for instance and needed typically in moems applications for seating optical fibres, the layers 60 and 61 are distributed unevenly on account of the etchings 12.
This produces an insufficient definition of the figure to be removed during the exposure and development, which makes the process practically impossible to use.
In particular, the etching 12 may reach a depth D of a few hundredths of a μm. Furthermore, if the die 51 is made of silicon, the etching 12 is often obtained by way of a chemical reaction which advances according to the crystallographic axes of the silicon, forming two walls 20 which produce an angle α=54.7° with respect to the x axis: the width W of the etching is therefore:W=2 D/tan α
If, for example, the etching concerns roughly one half of the thickness of a wafer of 625 μm, it reaches a depth D of about 300 μm; in this case, W=425 μm, a width that makes the above-mentioned unevenness extremely serious.
Moreover, choice of the positive photoresist is dictated by the first and second reasons already illustrated, and the layer of positive photoresist is normally applied in the liquid state: this favours unevenness of the deposition local to the etchings.
A second technical problem also exists: in some subassemblies, such as for instance those made in the moems technology, etchings 12 must be made on the same face as that containing other films. If the etching is made after the films are deposited, it is necessary to protect the films during the chemical reaction on the silicon, which is highly aggressive as it uses KOH or TMAH for numerous hours at a temperature of roughly 80° C., as is known to those acquainted with the sector art.
It is therefore advantageous to make the etchings in the silicon at the start of the process, for instance through a known process with a mask of SiO2, and to deposit and define the films at a later time, but in this way the first of the problems outlined crops up again.